Dragon Ball Z:

Name: Broly: The Legendary Super Saiyan.

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Broly is simply the most powerful Saiyan to ever live. As only an infant Broly had a power level of over 10,000, compared to Goku's level of 2. Broly's father Paragus became increasingly worried about Broly's inability to control his rage, so Pargus had a scientist create a necklace capable of restraining Broly. This was attached to him in his sleep for obvious reasons.
Note: Goku and Broly shared a special bond as they were born on the same day.
As an infant Broly became upset at Goku's constant crying, even though he was so much more powerful. This rage broke the necklace and Broly was released from his fathers control.Gokus friends had to channel their energy into Goku to help him defeat Broly, but he only fell into a coma for a short while.
Once awakened, Broly tried to seek his revenge on Goten (Goku's son), mistaking him for Goku. It was only after spiritual help from Goku that Gohan and Goten were able to finally kill Broly. This was achieved with a dual Kamehameha energy attack.
Broly had been killed but it was not the last of him, he regenerated through genetic engineering from the most intelligent scientists. He was brought back to life to seek his revenge to finally be defeated by everyone.

Broly is considered a villain because he has tremendous power and tries to destroy the world as he has done in the past.
Broly possess tremendous abilities that overpower the other saiyans.

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